#1 PROBLEM: POOR DISPLACEMENT OF MUD
MECHANISM: The volumetric fraction of the mud removed from the wellbore annulus by the cement slurry is called displacement efficiency. High displacement efficiencies increase the probability that the set cement will not contain channels of mud or that the cement will not have channeled through the mud. Satisfactory displacement efficiencies depend on many factors; however, the type of flow regime in which the cement slurry and the mud being displaced is flowing during displacement is dominant.
SOLUTION: The recognized flow regimes are (1) plug, (2) laminar and, (3) turbulent. The dominant solution to poor mud displacement is cement hydraulics. Other aids are pipe rotation and reciprocation. Mobil Oil showed rotation speeds of 35 rpm are sufficient. Exxon showed 80% standoff with centralizers is sufficient with proper cement and hydraulics.
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